Nobuyuki Soyama

Preparation of Bi-Based Ferroelectric Thin Films by Sol-Gel Method

The effects of Bi/Ta and Sr/Ta mole ratios on ferroelectric properties of Srx Biy Ta2Oz [ SBIT(x/y/2.0); 0.7≤x≤1.0, 2.0≤y≤2.6] thin-film capacitors were investigated. The SBIT films were grown by the sol-gel method using a spin-on coating on Pt/Ta/SiO2/Si and Pt/Ti/SiO2/Si substrates. The films were annealed at 800° C for one hour in oxygen atmosphere. Remanent polarization (Pr) depended strongly on the Sr/Ta mole ratio, and increased with decrease of the Sr/Ta mole ratio. On the other hand, Pr was almost independent of the Bi/Ta mole ratio. Scanning electron microscopy (SEM) images show that crystal grains grew with decrease of the Sr/Ta mole ratio. Electron probe micro analyzer (EPMA) analysis revealed that Bi content in SBIT films increased with decrease in Sr composition from stoichiometry.

Surface morphology of lead-based thin films and their properties

Surface morphology of lead-zirconate-titanate [PZT(52/48)] thin films prepared by sol-gel processing on Pt/Ti/ SiO2/Si substrates was studied. When the atomic ratio [Pb/(Zr+Ti)] of PZT gel films was 1, rosette structure was observed in the films after annealing at 600° C for one hour. The crystal structure of the rosette was identified as perovskite and that of the other area was nonperovskite, by electron diffraction analysis. Lead deficiency in the non-perovskite phase caused by lead diffusion into the bottom electrode was detected by energy dispersive X-ray spectroscopy and Auger electron spectroscopy. In order to prepare PZT films with a smooth surface, the following four means were efficient: addition of lead excess, rapid thermal annealing (RTA), adoption of buffer layer, and preparation of crystal nucleus on the substrate.

Influence of buffer layers on microstructural and ferroelectric characteristics of sol-gel derived PbZrxTi1-xO3 thin films

Pb(Zr, Ti)O3 (PZT) films with very thin PbTiO3 (PT) and ( Ba0.5Sr0.5)TiO3 (BST) buffer layers inserted between the film and Pt/Ti/ SiO2/Si substrate were synthesized by the sol-gel technique, and effects of variation in thickness upon microstructural and ferroelectric characteristics were investigated. Insertion of buffer layers was found to exert marked effects. PT buffer layers with thicknesses as thin as 2–4 nm yield PZT films with dense, homogeneous microstructure and good ferroelectric properties such as large remanent polarization, small coercive field and low leakage current densities. On the other hand, use of (2–4)-nm-thick BST buffer layers yields PZT films with dense microstructure and good dielectric properties such as increased permittivity, small coercive field and good leakage current characteristics. However, fatigue behavior of PZT films was not improved through insertion of PT or BST buffer layer.

Evaluation of Pb(Zr, Ti)O3 Films Derived from Propylene-Glycol-Based Sol-Gel Solutions

Crack-free PbZr0.52Ti0.48O3 (PZT) films up to almost 1 µm thick have been prepared on Pt/Ti/SiO2/Si substrates from stable propylene-glycol (diol)-based sol-gel solutions by a single coating. We have studied the film thickness dependence of various properties such as microstructure, crystal orientation, ferroelectric properties, and leakage current density for the PZT single-coated films. It was found that the 0.22-µm-thick PZT single-coated film was a dense film with (111)-orientation and exhibited good ferroelectric properties. In order to thicken the PZT dense film, we have studied a sol-gel technique involving multiple coatings of 0.22-µm-thick layers. Finally, the 0.66- and 1.10-µm-thick PZT multicoated films have been prepared on platinized silicon substrates by three and five coatings of 0.22-µm-thick layers. Various properties of the multicoated films have also been evaluated.

Micro-Patterning of PbZrxTi1-xO3 Thin Films Prepared by Photo Sensitive Sol-Gel Solution

The photo sensitivity of sol-gel solution of PbZr x Ti 1-x O 3 (PZT) was studied. A coated film of the sol-gel solution on Pt/Ti/SiO 2 /Si substrates was exposed to an excimer laser and developed with water. The film was finally annealed at 700 o C for 60 s by rapid thermal annealing (RTA). Ferroelectric perovskite phase was observed in the PZT thin films. The 130-nm-thick film showed P r of 11.2 μzC/cm 2 and E c of 93.8 kV/cm. From this process, half-micron patterns of PZT films were obtained

Preparation and Evaluation of Pb(Zr, Ti)O3 Thin Films for Low Voltage Operation

Pb(Zr, Ti)O3 (PZT) thin films with thickness of 120 nm were prepared on Pt/SiO2/Si substrate by using two types of sol–gel solutions. The film from modified solution consisted of uniform and small grain of perovskite phase, whereas the film from conventional solution consisted of large grain of perovskite phase and secondary phase. The film from modified solution had higher nucleation density for crystallization. That was why the different morphology between those two films occurred. The P-E hysteresis of the film from modified solution was well saturated at low applied voltage. Pr and Ec of the PZT(40/60) were 29.1 µC/cm2 and 58.3 kV/cm, respectively. This modified PZT sol–gel solutions enable to get thinner films, down to 120 nm, without degradation of the morphology and the properties.

Low-Temperature Crystallization of Sol-Gel Derived Pb(Zr0.4,Ti0.6)O3 Thin Films.

We have studied the crystallization of sol-gel derived Pb(Zr0.4Ti0.6)O3 [PZT(40/60)] thin films at 435 down to 420°C. The PZT(40/60) films were prepared at these temperatures on Pt/SiO2/Si substrates by a combination of diol-based solutions and modified film preparation processes. Various properties of the PZT(40/60) films such as microstructures, crystal orientation, ferroelectric properties, relative permittivity, and leakage current density were evaluated. It was found that PZT(40/60) films could be crystallized at 435 to 420°C by the diol-based solutions and the modified film preparation processes. The PZT(40/60) films had microstructures with perovskite-single-phase fine columnar grains and good electric characteristics such as remanent polarization (Pr) of 12 to 15 µC/cm2, relative permittivity (er) of 780, and breakdown voltage of more than 10 V.

Preparation of Dielectric Thin Films from Photosensitive Sol-Gel Solution

Fine-patterned Bax Sr1-x TiO3 (BST) thin films were successfully fabricated from a photosensitive sol-gel solution by means of UV irradiation. This solution was prepared from barium acetate, strontium acetate, and titanium isopropoxide. BST gel films on substrates formed from the solution by spin-coating were irradiated with UV rays through a mask pattern and developed with a mixture of ethanol and water. The obtained patterns were a negative of the mask. The films were finally annealed for crystallization. The obtained films had perovskite structure and good properties.

Characterization of self-patterned SrBi2Ta2O9 thin films from photo-sensitive solutions

Abstract Self-patterned SrBi2Ta2O9 thin films were successfully fabricated from photo-sensitive solutions by means of UV irradiation through photo masks. After conventional baking and wet etching the films were annealed. The photo-sensitive SrBi2Ta2O9 solutions give high resolution negative-pattern of the mask image down to 1 μm line width by deep UV irradiation at 900 mJ/cm2. The capacitor characteristics of the 210 nm thick films fabricated on the Pt/Ti/SiO2/Si substrates by this process showed 2Pr values of 17 μC/cm2, 2Ec of 89 kV/cm, and leakage current densities of 5×10−9 A/cm2 at 5 V. The films showed no fatigue after 1×1011 switching cycles.

Preparation of Pb(Zr,Ti)O3 films on Pt/Ti/Ta electrodes by sol-gel process

PbZr x Ti 1-x O 3 (PZT) films were prepared on Pt, Pt/Ti or Pt/Ti/Ta electrodes by the sol-gel process using rapid thermal annealing (RTA). In the case of Pt and Pt/Ti electrodes on poly-Si, thermal treatments of PZT films gave rise to interdiffusion between Pt and Si. This exerted an unfavorable influence on the preparation of PZT films with perovskite structure. The barrier effect of Ta films and the effect of Ti films on the crystallization of PZT films were investigated by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The electrical properties such as dielectric constants and P-E hysteresis curves were measured for PZT films with various Zr/Ti ratios on Pt/Ti/Ta/SiO 2 /Si(100) substrates